Orthogonal frequency-division multiplexing (OFDM) is a frequency domain multiplexing (FDM) scheme that uses multiple sub-carriers to deliver information in parallel in a frequency domain. In OFDM, a large number of closely-spaced orthogonal sub-carriers are used to carry data. The carriers are spaced apart in predetermined frequencies, which results in the orthogonality between carriers.
OFDM has been thought of as an attractive solution to high data-rate wireless transmission. This is mainly due to its parallel transmission mechanism in order to conform to a bandwidth-limited wireless channel. On the other hand, OFDM systems are more sensitive to frequency offset than single carrier system because frequency offsets can lead to the loss of orthogonality between the sub-carriers and, consequently, introduce inter-carrier interference (ICI). Methods to detect and compensate the frequency offset are always an area of concern in OFDM studies.
In detecting and compensating the frequency offset there are three principle concerns, accuracy, range, and complexity. Accuracy refers methods ability to correctly determine the carrier frequency offset. Range is typically normalized by the sub-carrier spacing, namely the maximum and minimum offset amounts for which the method can calculate an offset. For example, a method that is ±0.5 can only estimate an offset less than or equal to one half a sub-carrier spacing. Finally, complexity refers to the resources required to determine an offset. If a method is too complex, then too many resources or too much time is required to determine a frequency offset. In the later situation, a frequency offset that cannot be computed in adequate time is useless.
The following methods have been presented to estimate carrier offset. Moose presented a maximal likelihood estimation method for burst transmission OFDM systems in P. H. Moose, “A technique for orthogonal frequency division multiplexing frequency offset correction,” IEEE Trans. Commun., vol. 42, pp 2908-2914, October 1994, which is incorporated by reference herein. It compares two consecutive identical OFDM symbols to extract the phase rotation caused by frequency offset and the estimated range can reach ±0.5 sub-carrier spacing. Based on this, Schmidl et al adopts two identical halves within one OFDM symbol and extends estimate range to ±1 sub-carrier spacing but with less accuracy than Moose. T. M. Schmidl and D. C. Cox, “Robust frequency and timing synchronization for OFDM,” IEEE Trans. Commun., vol. 45, pp. 1613-1621. Morelli introduces a multi-stage estimate scheme with improved accuracy at the cost of increased computation. M. Morelli and U. Mengalli, “An improved frequency estimator for OFDM applications,” IEEE Communications Letters, vol. 3, pp. 75-77. Morelli's large increase in complexity prevents it from being commercially feasible in the real systems.
One significant contributor to frequency offset is the Doppler effect. Frequency offsets caused by a Doppler shift leads to the loss of orthogonality between sub-carriers, resulting in ICI. As the Doppler effect is a function of the velocity of the receiver as it moves relative to the transmitter, the effect of Doppler shifts will become more prevalent as wireless OFDM receivers become more mobile. As wireless receivers become increasingly mobile, the demand for an accurate, efficient, and cost-effective method of estimating frequency offset will become apparent.
The majority of the current offset estimation methods assume little or no mobility of the receiver. As focuses shift to mobile vehicular communication, however, new methods are needed to deal with Doppler shifts and the effects of waves bouncing off other vehicles or objects.
In this disclosure, a wide range estimator is presented with satisfied accurate and tolerable computation for OFDM systems based on Institute of Electrical and Electronics Engineers (IEEE) 802.IIa/g, IEEE 802.11p and IEEE 802.16 standards. IEEE 802.11a/g, IEEE 802.11p and 802.16 are herein incorporated by reference.
This section provides background information related to the present disclosure which is not necessarily prior art.